1. Field of the Invention
The present invention relates to an interchangeable telephoto lens system, for a single lens reflex camera and more particularly to a telephoto lens system which is capable of continuously effecting infinity distance to a high magnification proximity distance having a focal length of about 100 mm.
2. Description of the Prior Art
So-called macrolenses are known as lenses for short distance photography. In conventional macrolenses, the entire lens system is shifted along the optical axis in its focusing operation. However, in accordance with the development of large aperture macrolenses having focal lengths of about 50 mm and telephoto macrolenses having focal lengths of about 200 mm, various other focusing systems including so-called floating systems, internal focusing systems and rear focusing systems are being proposed. More specifically, various floating systems are applied to large aperture macrolenses having focal lengths of about 50 mm and a F-number of about F/2.8, mainly in order to compensate coma aberration in a short distance focusing condition. On the other hand, internal focusing systems are applied to telephoto macrolenses having focal lengths of about 200 mm for decreasing the lens shifting distance from an infinity focusing position to proximity the focusing position. The reason is that the lens shifting distance reaches about 200 mm if entire the lens system is shifted from an infinity focusing position to a proximity focusing position of high magnification of about .beta.=-1.0.
In the telephoto macrolenses having focal lengths about 100 mm, a proposal for designing larger aperture telephoto macrolenses with a decreasing lens shifting distance in the focusing operation is disclosed in U.S. Pat. No. 4,392,724. In the proposal, a floating system is applied for extending the aperture, and the lens shifting distance is reduced by increasing the refractive power of the shiftable lens group by locating a negative lens group in the image side of the shiftable lens group. In such a construction, however, the focal length of the whole lens system becomes shorter with an increasing magnification. Therefore, the advantage of the telephoto macrolenses as compared with the standard macrolenses, such as larger working distance, diminished perspective distortion and so on, is reduced. Thus in the proposed telephoto macrolenses these remains the problem that high magnification of .beta.=-1.0 can not be accomplished, or the above advantage is lost while accomplishing high magnification of .beta.=-1.0.